1. Field of the Invention
The present invention relates generally to telephone data transmission systems, and more particularly to a time independent data transmission system which allows a central computer, for example, to access at any time, any one of a plurality of remote data transmitting devices, such as electronic publication storage devices, hybrid telephone-cable television polling systems, computer data base subscribers, data sets, or a plurality of utility or consumption meters, via nondedicated telephone lines and which enables communication with the data devices without interfering with normal use of the subscriber's telephone. The present invention finds application in many areas, one of which, for instance, might be a shop by computer system where it might be necessary to transmit information to and from a plurality of data devices connected to information storage devices. Another might be the field of power demand data retrieval, wherein a central computer has a capability of accessing a plurality of remote data devices which have stored therein data corresponding to the usage level of, for example, electrical power, water usage, or gas usage at the remote location. Still another might be remote control of heating, ventilating and air-conditioning equipment through telephone lines. Although the invention will be described for use with telephone lines, it will be apparent to one skilled in the art that the invention can also be used with transmission media other than telephone lines.
2. Description of the Prior Art
Various systems are presently known which allow central computers to access individual subscriber's data sets via non-dedicated telephone lines. For example, see U.S. Pat. Nos. 4,126,762, 4,104,486, 4,394,540, 4,345,113 and 4,469,917. In the '486 and '540 patents, the telephone is isolated from the telephone line during a selected time window. In the '762 patent, in one embodiment a time window is utilized. In another embodiment, sequential ringing signals having a different time spacing than signals from a normal telephone call condition the remote terminal to intercept the ringing signals and to provide selective connection of a data accumulator to the telephone line. It is preferable to use the subscriber's existing telephone line rather than install a separate dedicated line to provide access to the data set. Thus, systems of this type require that the telephone line be available for both use by ordinary persons and by computers calling the subscriber. Requirements of systems of this type are that when computer calls are received the call is coupled with or automatically transferred to a data set and the telephone preferably should not ring.
In applicant's prior U.S. Pat. No. 4,345,113, an automatic telephone message interception system is described which permits the use of a single telephone line for both ordinary personal use and for receiving calls from a computer, wherein computer calls are automatically transferred to the subscriber's data set without ringing the subscriber's telephone. An ordinary non-computer call made to the subscriber, would, however, be answered by the automatic telephone message interception system described in this U.S. Patent even if no one were present at the subscriber's telephone. The caller would thus be charged for a brief call even though no one answered the phone.
The subscriber could, of course, solve this problem by shutting off the automatic system if the telephone is to be left unattended, but the data set could not be accessed by the computer while the sytem was shut off.
The need thus existed for a system which both allows a central computer to access the subscriber's data set via a non-dedicated telephone line and which allows normal operation of the telephone by both the subscriber and other persons calling the subscriber. In particular, such a system should allow a computer to access the subscriber's data set without ringing the subscriber's telephone and should allow persons to call the subscriber at any time without being charged for the call if the telephone is unanswered.
In applicant's prior U.S. Pat. No. 4,469,917, a system and apparatus is described which satisfies the above need. In that patent, however, a system and apparatus is described for connecting a central computer to a remote subscriber's data set using the subscriber's non-dedicated telephone line during a preselected window in time. In applicant's copending application Ser. No. 643,450, the apparatus described in U.S. Pat. No. 4,469,917 has been modified to permit connection of the central computer at any time to a remote subscriber's data set using the subscriber's non-dedicated telephone line and without interference of any sort with the normal use of the subscriber's telephone. In the system described in U.S. Pat. No. 4,469,917, the apparatus detects a ringing signal on the telephone line during a preselected window in time during which the subscriber's data set is connected to the telephone line and the subscriber's telephone is disconnected. The apparatus does nothing until the ringing signal stops, indicating that the caller has hung up. The apparatus then causes the data set to dial up the central computer and proceed to transmit or receive data during the time window.
In U.S. patent application Ser. No. 643,450, a system which allows a central computer to access a subscriber's data set via non-dedicated telephone lines at any time, i.e., without limitation to a time window, is described. This system allows normal operation of the telephone by the subscriber without interference in any way with the subscriber use of the telephone at any time, i.e., the subscriber's telephone does not ring when the central computer calls, the subscriber is allowed to use the telephone even if data transmission with the computer is occurring and outside callers are not charged for calls when the phone is unanswered.
The apparatus described in U.S. patent application Ser. No. 643,450, however, must be modified in order to allow its use in applications where more than one data sending or receiving device is located at the remote location. For example, in a preferred application of the present invention, a plurality of consumption level devices are to be interrogated at the remote location by a central computer. The device described in U.S. Ser. No. 643,450 must be modified in order to allow the central computer to read signals from the proper data device, which might be, for example, a selected one of electric, water and gas utility meters. In the device described in Ser. No. 643,450, the telephone instrument is normally disconnected from the telephone line. The central computer issues a ringing signal having a single ring burst, and the device described in that patent application detects the single ring burst and triggers a data set to provide data over the telephone line to the central computer. If a ringing signal with more than one ring burst is detected, then the device recognizes that the call is not from the central computer and disconnects the data set from the telephone line and connects the telephone instrument to the telephone line, enabling the user to answer the call. Because the telephone was disconnected if a ringing signal with only one ring burst is detected, the telephone subscriber never hears a call from the central computer.
The device described in U.S. Ser. No. 643,450 provides a very advantageous solution to the problem of accessing a remote data set. However, since the central computer in that application only transmits a single ringing signal, and the data set is triggered by the device to transmit data in response to the ringing signal having the single ring burst, the system does not provide means for accommodating a remote location at which more than one data device, for example, a plurality of consumption meters, are disposed, since no means are provided for accessing the data from a particular one of the data devices or consumption meters.
Accordingly, the need exists for a data transmission system which allows a central computer to access any one of a plurality of data devices disposed at the remote location.